Magnetic resonance imaging system

ABSTRACT

A magnetic resonance imaging (MRI) system obtains an MR image of an object. The system detects an ECG signal and performs a pulse sequence of RF gradient magnetic fields toward the object. Imaging defined by the pulse sequence is longer in temporal length than one heartbeat. The system further acquires an MR signal from the object in response to performance of the pulse sequence and produces the MR image based on the acquired MR signal. Also possible are: a plurality of divided MT pulses instead of the conventional single MT pulse, an SE-system pulse sequence having a shorter echo train spacing, and the generation of sounds by applying gradient pulses incorporated in an imaging pulse sequence so as to automatically instruct a patient to perform an intermittent breath hold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of copending Ser. No. 13/283,948 filed Oct. 28, 2011,which is a continuation of Ser. No. 11/819,655 filed Jun. 28, 2007 (nowU.S. Pat. No. 8,131,338 issued Mar. 6, 2012), which is a division ofSer. No. 10/635,685 filed Aug. 7, 2003 (now U.S. Pat. No. 7,254,437issued Aug. 7, 2007), which is a CIP of Ser. No. 09/293,062 filed Apr.16, 1999 (now abandoned), all of which claim priority from JapaneseApplication 108256/1998 filed Apr. 17, 1998, and Japanese ApplicationNo. 54614/1999 filed Mar. 2, 1999, the entire disclosures of all ofwhich are incorporated herein by reference. This application is alsorelated to Ser. No. 10/024,536 (now U.S. Pat. No. 7,308,298 issued Dec.11, 2007).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to magnetic resonance imaging forobtaining blood vessels and tissue of a subject (patient) on the basisof a magnetic resonance phenomenon occurring in the subject. Moreparticularly, this invention is concerned with a magnetic resonanceimaging (MRI) system and magnetic resonance (MR) imaging method thatprovides tissue/blood contrast images of a higher quality.

In one aspect, such images are provided by utilizing MT (magnetizationtransfer) pulses that are able to greatly raise contrast between blood(or flow of blood) and tissue. In another aspect, such high-qualitytissue/blood contrast images are obtained by acquiring a plurality ofecho signals responding to one piece of exciting pulse incorporated in apulse sequence to which both a degree of resolution and a time for echoacquisition are optimized. In this later case, a patient's breath holdis improved by using an easier self-navigation technique.

The term “blood” (or “flow of blood”) is used to mean a representativeof such fluid as cerebral spinal fluid (CSF), blood (or flow of blood),or the like.

2. Description of Related Art

Magnetic resonance imaging (MRI) is a technique for magneticallyexciting nuclear spins of a subject positioned in a static magneticfield by applying a radio-frequency (RF) signal with the Larmorfrequency, and reconstructing an image using MR signals induced by theexcitation.

One of the magnetic resonance (MR) imaging fields is MR angiography. Aphase contrast method is one technique for the MR angiography, whichuses pulses referred to as flow encoding pulses. Another method for theMR angiography is to utilize MT effects (or may be referred to as MTC(magnetization transfer contrast effects)) to produce images in whichcontrast between blood (flow of blood) and tissue is given. Thistechnique has frequently been used lately. One such example is disclosedby U.S. Pat. No. 5,050,609 (Magnetization Transfer Contrast and ProtonRelaxation and Use Thereof in Magnetic Resonance Imaging).

The research of MT effects originates from the study of an ST(saturation transfer) method by Forsen & Hoffman (refer to Forsen etal., Journal of Chemical Physics, Vol. 39(11), pp. 2892-2901 (1963)).The MT effects are based on chemical exchanges and/or cross-relaxationbetween protons of a plurality of types of nuclear pools, such as freewater and macromolecules.

As conventional MR angiography that uses MT effects, there are proposedseveral techniques as below.

In each of FIGS. 1A-1C, the left-side graph is a frequency spectrum offree water and macromolecules, while the right-side graph illustratesthe exchange and relaxation relation of their magnetizations Mr and Mf.As shown in the spectra of protons of free water and macromolecules, thefree water of which T2 (spin-spin) relaxation time is longer (T2 isapprox. 100 msec) and the macromolecules of which T2 relaxation time isshorter (T2 is approx. 0.1-0.2 msec) resonate in the same frequencyrange. Since the T2 relaxation time of a free water signal is longer,its Fourier-transformed signal has a peak of which a half-width value isnarrow, as shown therein. However, in the case of the signal of protonswhose movement is restricted among macromolecules, such as protein, itsFourier-transformed signal shows the broader half-width value, due to ashorter T2 relaxation time, no longer appearing as a distinct peak inthe spectra.

When taking the resonance peak frequency f₀ of free water as the centerfrequency, a frequency-selective pulse serving as an MT pulse is appliedto excite a frequency range shifted, for example, by 500 Hz from thecenter frequency f₀ of free water (that is, off-resonance excitation),as shown in the left side of FIG. 1B. This excitation causes themagnetization Hf of free water and those Hr of macromolecules, both ofwhich are in equilibrium as shown in the right side of FIG. 1A, to themagnetization Hf of free water moves to those Hr of macromolecules asshown in the right side of FIG. 1B. As a result of it, as illustrated inthe left-side graph of FIG. 10, the signal value of protons of freewater decreases. Differences in signal values are caused between oneregion in which the chemical exchanges and/or cross-relaxation betweenfree water and macromolecules are reflected and the other region inwhich such chemical exchanges and/or cross-relaxation is not reflected.These differences lead to differences in contrast between flow of bloodand tissue, providing blood flow images.

At present, the MR angiography based on MT effects is classified intospatially non-selective imaging and slice-selective imaging.

As an example of the former, known is G. P. Pike, MRM 25, pp. 327-379,1992, in which a frequency-selective binomial pulse is used as the MTpulse and applied in the spatially non-selective manner. Contrastbetween parenchyma and flow of blood is obtained according to a relationof “MT effects of parenchyma>MT effects of flow of blood.”

On one hand, as an example for the latter imaging, there is proposed M.Miyazaki, MRM 32, pp. 52-59, 1994. This paper teaches that aslice-selective MT pulse is composed by an RF excitation pulse of whichapplication time is long and gradient spoiler pulses. And, applicationsuch MT pulse causes MR signals emanated from stationary parenchyma in aslice to be imaged to be lowered largely than flow of blood that passestherethrough, due to its MT effects, as well as MT effects received byflow of blood that comes into the slice to decease (but degrees ofsignal decrease of flow of blood are less than that of parenchyma). Thisprovides contrast between flow of blood and parenchyma.

However, in the case of the foregoing MR angiography making use of theslice-selective MT pulse, blood flowing into a slice to be imagedsuffers a considerably large amount of MT effects, because a flip anglegiven magnetization when the MT pulse is applied is set to a greateramount (for example, 500-1000 degrees). This results in that MR signalsvalue emanated from the blood passing the slice decrease largely.Therefore, contrast between blood and parenchyma is not always fullysatisfied under recent needs for higher resolution of images.

In addition, in the field of MRI, another clinically significant imagingtechnique is T2-weighted imaging that emphasizes the T2 relaxationphenomenon. To perform this imaging requires that the repetition time TRbe longer. The entire scanning takes as long as 10 minutes, for example,imposing a greater burden on a patient. To improve this, FSE (Fast SpinEcho) and EPI (Echo Planar Imaging) methods are proposed that produce aplurality of echo signals in response to one excitation pulse.

The EPI method is used to switch a readout gradient between the twopolarities to produce field echoes consecutively. This enablessingle-shot imaging.

The FSE method is characteristic of using a plurality of refocus pulsesthat are applied after the application of one excitation (shot) pulseand produce multiple echoes. Compared to the EPI method, the FSE methodneeds a loner scanning (imaging) time, but possesses various advantages,such as higher resistance to the non-uniformity of a static magneticfield. Therefore, the FSE method of which number of shots are increasedand the effective TE (Echo Time) is shortened has been widely used forproviding clinical effectiveness.

On one hand, for depicting the cardiac vascular systems, thesynchronization with the cardiac temporal phase, which is typicallyrepresented by an ECG signal, is unavoidable. In addition,field-echo-system pulse sequences whose repetition time TR and/or echotime TE are shortened are used for the reduced scanning time in imagingthe cardiac vascular systems. Particularly, a segmented FFE (Fast FE)can raise a temporal efficiency for scanning, which has been usedwidely. In the FFE sequences, if the TR or TE is reduced, image contrastis lowered. Thus, to compensate this lowered contrast, an MT pulseand/or fat-suppression pulse are preferably used. In addition, whenconsidering the fact that three-dimensional imaging takes a long timefor scanning, it is, in fact, impossible to force a patient to continueto hold his/her breath for such a long scanning time. Continuous imagingis, therefore, executed in synchronism with an ECG signal. In thisECG-gating imaging, a problem that the heart moves with respirationarises. One solution to respiratory motions is selection or correctionof data using navigator echo produced by a navigation pulse incorporatedin an imaging pulse sequence.

In general, in cases it is desired to quickly obtain T1-weighted imagesor PD (proton density)-weighted images, FE-system pulse sequences aresuperior to others. On the contrary, if T2-weighted images whosesensitivity for lesions is excellent, pulse sequences that is capable ofacquiring a plurality of echo signals in response to one time ofexcitation are effective, increasing efficiency in data acquisition.Particularly, for T2-weighted images whose TR or TE is elongated, theEPI or FSE method is preferable and allows the total scanning time to bereduced and image artifacts to be suppressed.

However, due to the fact that the conventional T2-weighted imagesobtained with the EPI or FSE method are based on a multi-slicetechnique, a signal from blood inflowing into a certain slice to beimaged has already been saturated by the RF excitation for other slices.Hence, this T2-weighted imaging provides blood signals of lessintensities, being inappropriate for the depiction of flow of blood.

A report has been made that imaging T2-weighted imagesthree-dimensionally makes it possible to depict a vascular systemutilizing the characteristic that a region where blood flows slowly islonger in the T2 relaxation time than a parenchyma region. However, evenwhen using this reported technique, images of a sufficient contrastbetween blood and tissue are still unavailable, independently of themagnitudes of flow of blood. In this three-dimensional imaging, when thenumber of slices excited per unit time is reduced, there is a tendencythat it becomes difficult to reduce signals from muscle or the liver,the signal reduction resulting from MT effects according to the FSEmethod. Thus, the contrast between such tissue and blood is lowered.

As described above, 3D imaging with a continuous breath hold is normallydifficult or impossible. In the conventional 3D MR imaging or 3D MRangiography for the abdomen, an intermittent breath hold may have beencarried out to reduce artifacts due to respiratory body motions. Arespiration-gating technique may have been used for achieving timing toa patient's respiration.

However, since there is no steady way of informing a patient of thetiming of the respiration, this respiration gating has not beenpopularly used in normal clinical fields. An operator needs to instructa patient to hold the breath at imaging sites each time of theintermittent breath holds. Thus, operative burdens to the operator willbe increased. There is a large possibility that respiration timing failsand image quality becomes poor.

BRIEF SUMMARY

The present invention attempts to break through the foregoing currentsituations.

An object of the present invention to largely increase contrast betweenblood and parenchyma compared to the prior method by decreasing theinfluence of MT effects given blood, so that a higher depiction bloodflow image or parenchyma image is gained in imaging by which adistinction is made by MT pulses between blood in motion and stationaryblood and/or parenchyma.

Another object of the present invention is to provide T2-weighted imagesthat have tissue contrasts as high as obtained by FE-system pulsesequences or more as well as have fewer artifacts, thereby beingclinically effective.

More practically, the object is to obtain T2-weighted cardiac vesselimages that have tissue contrasts between blood and cardiac muscleand/or cardiac wall as high as obtained by FE-system pulse sequences ormore, as well as to have fewer artifacts.

Still another object of the present invention is to control tissuecontrasts by controlling both preventing signals from being lowered andMT effects gained.

Still another object of the present invention is to reduce artifacts dueto an ECG-gating technique by employing a breath-hold technique togetherwith the ECG gating.

Still another object of the present invention is to steadily inform apatient of the start timing of an intermittent breath hold, therebyproviding images from which artifacts resulting from the patient'srespiratory body motions are largely eliminated.

For accomplishing the above objects, a first configuration according tothe present invention is provided by A magnetic resonance imaging (MRI)system providing an MR image of an object to be imaged, comprising:means for detecting a signal indicative of cardiac temporal phases ofthe object; means for performing a pulse sequence toward the object, aunit of imaging defined by the pulse sequence being longer in temporallength than one heartbeat represented by the detected signal; means foracquiring an MR signal from the object in response to performance of thepulse sequence; and means for producing the MR image based on theacquired MR signal.

According to a second configuration of the present invention, there isprovided a magnetic resonance imaging (MRI) system providing an MR imageof a region to be imaged of an object, comprising: means for applying anMT (magnetization transfer) pulse of which frequency is different than afrequency specifying the region to be imaged; means for applying agradient spoiler pulse after the MT pulse applied; means for scanningthe region to be imaged with a pulse sequence to cause an MR signal fromthe region; and means for producing the MR image using the MR signal.

Preferably, the MT pulse applied by the MT pulse applying means consistsof a plurality of divided MT pulses. It is also preferable that theplurality of divided MT pulses applied by the MT pulse applying means iscomposed of a plurality of RF (radio frequency) pulses appliedslice-non-selectively.

It is further preferred that each of the plurality of divided MT pulsesapplied by the MT pulse applying means is an RF pulse exciting spinsresiding in a slice region determined by a frequency thereof, and thesystem comprises means for applying a gradient pulse, appliedconcurrently with the RF pulse, for selecting the slice region. In thiscase, each RF pulse has a shorter applied duration and is given asmaller flip angle for exciting spins of the slice region. It ispreferred that the gradient spoiler pulse applying means apply thegradient spoiler pulse in at least one of slice, readout andphase-encoding directions spatially set toward the object.

According to a third configuration of the present invention, there isprovided a magnetic resonance imaging method of providing an MR(magnetic resonance) image of a region to be imaged of an object; themethod comprising the steps of: applying an MT (magnetization transfer)pulse of which frequency is different than a frequency specifying theregion to be imaged; applying a gradient spoiler pulse after the MTpulse applied; scanning the region to be imaged with a pulse sequence tocause an MR signal from the region; and producing the MR image using theMR signal.

According to a fourth configuration of the present invention, there isprovided a magnetic resonance imaging method of acquiring from an objectan MR (magnetic resonance) signal based on a magnetic resonancephenomenon of at least two kinds of nuclear pools coupled with eachother by at least one of a chemical exchange phenomenon or across-relaxation phenomenon in the object, comprising the steps of:applying, in turn, a plurality of divided MT pulses to a region selectedin the object, thereby decoupling the coupling of the at least two kindsof nuclear pools; applying a gradient spoiler pulse to the decouplednuclear pools; and scanning, with a pulse sequence, another region to beimaged different in position from the MT pulse applied region as well asacquiring the MR signal from the another region.

Thus, pluralities of divided MT pulses, each of which have a shortduration and are assigned to a small flip angle, are applied to a regiondifferent from another region to be imaged, and the imaging region isexcited in an off-resonance state. The divided MT pulses reduce theapparent T1 relaxation time of blood that is flowing. The spins of bloodthat are flowing are T1-relaxed at faster speeds and quickly returned totheir steady states.

In other words, MT effects given free water of flow of blood arelowered, the total MT effects become small. Compared to the conventionalMT pulse, signals from blood that has inflowed into the imaging regionare higher. On the other hand, because the parenchyma of the imagingregion is stationary or can be regarded as being stationary, the dividedMT pulses simply give the parenchyma the sum of MT effects thatcorrespond to the MT pulses. This amount of the MT effects reducedlargely as expected.

In consequence, in comparison with the conventional one, the contrastbetween the blood and parenchyma in an MRA image of this imaging regionis improved by an amount that corresponds to the apparently reduced T1relaxation time against flowing blood, resulting in excellent depictionability.

According to a fifth configuration of the present invention, there isprovided a magnetic resonance imaging (MRI) system providing an MR imageof a region to be imaged of an object, comprising: means for performinga pulse sequence including a pre-sequence for applying an MT(magnetization transfer) pulse causing MT effects in spins of the objectand an SE (spin echo)-system data acquisition sequence, which followsthe pre-sequence, for generating a plurality of echo signals in responseto one time of excitation of the spins using an RF (radio frequency)magnetic field; means for acquiring the plurality of echo signals; andmeans for producing the MR image based on the acquired echo signals.

Preferably, the pulse sequence is applied to either one of athree-dimensional volume region or a two-dimensional slab and formed toperform at least one time of excitation within a repetition time ofimaging.

Still preferably, the system further comprises means for detecting asignal indicative of cardiac temporal phases of the object; means forsynchronizing start of the pulse sequence with a reference wave of thedetected signal; and means for instructing the object to perform anintermittent breath hold according to the detected signal. For example,the detecting means detect an ECG (electrocardiogram) signal of theobject as the signal indicative of cardiac temporal phases thereof. Itis preferred that the breath-hold instructing means include a gradientpulse incorporated into the pulse sequence, the gradient pulsegenerating a sound in a gantry of the MRI system when applied.

A sixth configuration of the present invention is that a magneticresonance (MR) imaging method of providing an MR image of an object,comprising the steps of: performing a pulse sequence including apre-sequence for applying an MT (magnetization transfer) pulse causingMT effects in spins of the object and an SE (spin echo)-system dataacquisition sequence, which follows the pre-sequence, for generating aplurality of echo signals in response to one time of excitation of thespins using an RF (radio frequency) magnetic field, and producing the MRimage from the plurality of echo signals.

By the above single-shot imaging, artifacts due to motions in thephase-encoding direction can be suppressed steadily. In addition,artifacts occurring in the slice-encoding direction can be reduced byemploying in combination both the ECG gating and intermittent breathhold techniques. It is, therefore, possible to provide athree-dimensional image having higher value of resolution. When aT2-weighted image of a higher resolution value is obtainable, signalsfrom the cardiac muscle and others can be suppressed by the MT pulse andsignals from fat can also be suppressed by the fat-suppression pulse.This enables tissue contrasts between blood and the cardiac muscleand/or cardiac wall to rise greatly. Hence, an effective imagingtechnique for the heart is available.

A seventh configuration of the present invention is that a magneticresonance imaging system comprising: means for detecting an ECG(electrocardiogram) signal from the object; means for performing a pulsesequence to acquire MR data with an object, including means forsynchronizing start of the pulse sequence according to the ECG signal;means for instructing the object an intermittent breath hold accordingto the ECG signal; and means for producing an MR (magnetic resonance)image through acquiring MR data generated in response to the pulsesequence performed, wherein the breath-hold instructing means include agradient pulse incorporated into the pulse sequence, the gradient pulsegenerating a sound in a gantry of the MRI system when applied.

The remaining features of the invention will be clearly understood fromthe description of various preferred embodiments, which are describedwith accompanying drawings below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A-1C illustrate MT effects according to the conventional MTpulse;

FIG. 2 exemplifies in a block form an MRI system used in variousembodiments of the present invention;

FIG. 3 is a timing chart showing one example of pulse sequences used ina first embodiment;

FIGS. 4A and 4B show the positional relationship between one slice to beimaged and the other slice to be pre-excited for each diagnostic region;

FIG. 5 illustrates decoupling between spins;

FIG. 6 shows Tc-dependency of T1 relaxation time and others;

FIGS. 7A-7D illustrate MT effects on divided MT pulses against spins offree water and macromolecules of an object in motion;

FIG. 8 is part of a pulse sequence showing another example of applyingdivided MT pulses;

FIG. 9 is part of a pulse sequence showing another example of applyingdivided MT pulses;

FIG. 10 pictorially shows the temporary relationship of two times ofimaging according to a second embodiment;

FIG. 11 outlines a flowchart exemplifying procedures for the firstimaging, which are executed by the host computer;

FIG. 12 outlines a flowchart exemplifying imaging procedures with ECGgating;

FIG. 13 is a pulse sequence showing two times of imaging based on ECGgating;

FIG. 14 illustrates the relationship between a three-dimensional regionto be imaged and slice-encoding, phase-encoding and readout directions;

FIG. 15 is an outlined flowchart showing an example of data processingcarried out after echo acquisition;

FIGS. 16A-16C illustrate a weighted difference process carried out asone image-processing step;

FIGS. 17A-B are illustrations explaining differences in MT effectsbetween parenchyma and blood;

FIG. 18 is an outlined pulse sequence according to a third embodiment ofthe present invention;

FIG. 19 is a partial detailed pulse sequence shown in FIG. 18;

FIG. 20 illustrates an imaging region employed in the third embodiment;

FIG. 21 is an outlined pulse sequence according to a fourth embodimentof the present invention;

FIG. 22 is an outlined pulse sequence according to a fifth embodiment ofthe present invention;

FIG. 23 is an outlined pulse sequence according to a sixth embodiment ofthe present invention;

FIGS. 24A and 24B pictorially show a imaging region in the sixthembodiment; and

FIG. 25 is an outlined pulse sequence according to a seventh embodimentof the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS First Embodiment

A first embodiment of the present invention will now be described withreference to FIGS. 2-7.

FIG. 2 shows the outlined configuration of a magnetic resonance imaging(MRI) system in accordance with the embodiment of the present invention,which will be described below.

The MRI system comprises a patient couch on which a patient P lies down,static magnetic field generating components for generating a staticmagnetic field, magnetic field gradient generating components forappending positional information to a static magnetic field,transmitting/receiving components for transmitting and receiving aradio-frequency (RF) signal, control and arithmetic operation componentsresponsible for control of the whole system and for imagereconstruction, electrocardiographing components for acquiring an ECGsignal of a patient, which is a representative of signals indicative ofcardiac temporal phases of the patient, and breath hold instructingcomponents for instructing the patient to perform a breath hold.

The static magnetic field generating components includes a magnet 1 thatis of, for example, a superconducting type, and a static power supply 2for supplying a current to the magnet 1, and generates a static magneticfield H₀ in an axial direction (Z-axis direction) in a cylindrical bore(diagnostic space) into which the patient P is inserted. The magnet unitincludes shim coils 14. A current used to homogenize a static magneticfield is supplied from a shim coil power supply 15 to the shim coils 14under the control of a host computer to be described later. The couchtop of the patient couch on which the patient P lies down can beinserted into the bore of the magnet 1 so that the couch top can bewithdrawn.

The magnetic field gradient generating components includes a gradientcoil unit 3 incorporated in the magnet 1. The gradient coil unit 3includes three pairs (kinds) of x-, y- and z-coils 3 x-3 z used togenerate magnetic field gradients changing in strength in X-axis, Y-axisand Z-axis directions that are mutually orthogonal. The magnetic fieldgradient generator further includes a gradient power supply 4 forsupplying a current to the x-, y- and z-coils 3 x-3 z. The gradientpower supply 4 supplies a pulsating current used to generate a magneticfield gradient to the x-, y- and z-coils 3 x-3 z under the control of asequencer that will be described later.

The pulsating current supplied from the gradient power supply 4 to thex-, y- and z-coils 3 x-3 z is controlled, whereby magnetic fieldgradients changing in the three axial directions, that is, the X-, Y-and Z-directions are synthesized. Thus, directions in which a slicemagnetic field gradient G_(S), a phase-encoding magnetic field gradientG_(E) and a readout (frequency-encoding) magnetic field gradient G_(R)are applied can be specified and changed arbitrarily. The magnetic fieldgradients to be applied in a slice direction, a phase-encoding directionthat is a direction the distribution of spins in which is phase-encoded,and a readout direction that is a direction in which an MR signal isread are superposed on the static magnetic field H₀.

The transmitting/receiving components includes a radio-frequency (RF)coil 7 located in the vicinity of a patient P in the scanning spaceinside the magnet 1, and a transmitter 8T and a receiver 8R connected tothe coil 7. Under the control of a sequencer described later, thetransmitter 8T supplies RF current pulses with the Larmor frequency,which are used to excite spins to cause nuclear magnetic resonance(NMR), while the receiver 8R receives echo signals (RF signals) via theRF coil 7, and carries out various kinds of signal processing with theecho signals so as to produce a corresponding digital echo data.

Furthermore, the control and arithmetic operation components include asequencer 5 (often referred to as sequence controller), a host computer6, an arithmetic operation unit 10, a storage unit 11, a display unit 12and an input unit 13.

Among them, the host computer 6, which has a CPU and memories, has thefunction of providing sequencer 5 with information about a pulsesequence and managing the operations of the entire system, according toinstalled software programs. The host computer 6 also serves as anelement for instructing a patient to perform a breath hold by a voicemassage produced using an automatic voice synthesis technique or others.

Sequencer 5, which has a CPU and memories, stores pulse-sequenceinformation sent from the host computer 6, controls a series ofoperations performed by gradient power supply 4, transmitter 8T andreceiver 8R according to the stored information, and temporarilyreceives digital data corresponding to MR signals outputted fromreceiver 8R so as to transmit them to arithmetic operation unit 10.

The pulse-sequence information includes all information required foroperating the gradient power supply 4, transmitter 8T and receiver 8Raccording to a pulse sequence. Such information includes the strength,duration and application timing of pulsed currents applied to the x-, y-and z-coil 3 x-3 z.

As the pulse sequence, a two-dimensional (2D) scan or athree-dimensional (3D) scan can be adopted. Available pulse trains arean SE (spin echo) train, FE (field gradient echo) train, FSE (Fast SE)train, FASE (Fast asymmetric SE) train, and others.

In this MRI system, prior to a data acquisition sequence (may referredto as a main sequence), a pre-sequence including RF pulses functioningas MT pulses and gradient spoiler pulses applied to any one or morelogic-axis directions (slice, phase-encoding and readout directions) isapplied by sequencer 5 under control of host computer 6.

The MT pulses are set to be applied slice-selectively. In other words,as shown in FIG. 3, the MT pulses are made up of a plurality of RFpulses for excitation which are individually formed by a sin c function,for example, and each RF pulse (i.e., each MT pulse) is accompanied by aconcurrently applied slice gradient G_(S). The number of MT pulsesapplied is n-pieces (n is an integer larger than 1; for example, tenpieces). The flip angle of each RF (MT) pulse is less than that (largerflip angle, such as 500-1000 degrees) of the conventional MT pulse,being a smaller divided value (for example, 90-100 degrees). The MTpulses are formed by modulating with a sin c function RF pulses eachhaving a desired frequency offset. Applying those MT pulses will allowparenchyma and free water in a region to be imaged to suffer MT effects.Thus, as in one aspect, a signal from the parenchyma decreases largelythan that of free water, producing a larger contrast ratio between them.

The n-sets of the MT pulses and the slice gradients G_(S) are, in turn,applied before a gradient spoiler pulse is applied for dephasing spinsof the slice, phase-encoding and readout directions.

The arithmetic operation unit 10 receives digital echo data sent fromthe receiver 8R via sequencer 5, maps the data in a Fourier space (orthe k-space or frequency space) formed in an incorporated memory, andperforms a two-dimensional or three-dimensional Fourier transform withthe mapped data so as to reconstruct an image in the real space.Moreover, the arithmetic operation unit 10 carries out the synthesis ofreconstructed image data. The Fourier transform may be performed by hostcomputer 6, not by sequencer 5.

The storage unit 11 can preserve raw echo data and reconstructed imagedata. The display unit 12 displays an image, and can be used to input tothe host computer 6 desired information entered into the input unit 13by an operator; such as information about parameters for determining anECG-gating time, scan conditions, the type of pulse sequence and thetype of technique of image synthesis.

The voice generator 16, which composes a constituent of the breath-holdinstructing components, utters, for example, a voice message informing apatient of the start or end of breath hold in response to commands sentfrom the host computer 6. However, this generator 16 may be removedunless the breath-hold instruction is carried out or if it is carriedout by any other means.

Furthermore, the electrocardiographing components comprises an ECGsensor 17 attached to the patient body to detect an electric ECG signaland an ECG unit 18 performing various processes including digitizationwith the detected ECG signal and sending it to both host computer 6 andsequencer 5. This measured ECG signal is used by host computer 6 andsequencer 5 to perform an ECG-gating scan.

The operation of the MRI system according to this embodiment will bedescribed with reference to FIGS. 3-7.

First, a pulse sequence for MR angiography shown in FIG. 3 is executedin response to a command from sequencer 5.

As shown therein, the pulse sequence is composed of a pre-sequenceSQ_(pre) previously executed before each RF excitation and a dataacquisition sequence SQ_(acq) that follows the previous one.

The pre-sequence SQ_(pre) includes a train of MT pulses P_(MT) causingMT effects and a set of gradient spoiler pulses SP_(S), SP_(R) andSP_(E). The train of MT pulses P_(MT) consists of a plurality ofexciting RF pulses P₁, P₂, P₃, . . . , P_(n) applied in turn as MTpulses and a plurality of slice gradients G_(S) applied in parallel withthose MT pulses.

The slice gradients G_(S) have a strength level of G_(S1) that permits aselected slice S_(mt) to be positioned at a different location from aslice S_(ima), no gap or with a gap, as depicted in FIG. 4A or 4B.

Each MT pulse P₁ (P₂, P₃, . . . , P_(n)) is formed by a sin c function,for example, and its pulse strength is set so that the flip angle FAbecomes for example, 90 degrees. The total number of MT pulses P₁, P₂,P₃, . . . , P_(n) is ten, for example.

In this embodiment, instead of the conventional one MT pulse having alarge flip angle FA (for example, 500-1000 degrees) appliedslice-selectively, a plurality of divided MT pulses are appliedconsecutively as an MT pulse train.

A flip angle FA given each MT pulse P₁(P₂, P₃, . . . , P_(n)) is adivided value (preferably, 90-100 degrees) set so that collectively theentire MT pulse train is able to cause desired MT effects. The totalnumber of MT pulses is set at an appropriate number (for example, 5-10pieces) in consideration of MT effects given by the entire MT pulsetrain and the time required to complete an entire scanning (imaging)sequence. The duration of each MT pulse is divided to be as short asapprox. 1300 μsec, which is much shorter than the heretoforeconventional slice-selective MT pulse.

A period of Δt between the divided MT pulses is determined so that MTeffects for water/fat of parenchyma of a slice to which the MT pulsesare applied (refer to FIG. 4) can be optimized. This period Δt dependson regions to be imaged and, if necessary, Δt=0 can even be set.

The gradient spoiler pulses SP_(S), SP_(R) and SP_(E) to be applied inthe slice, phase-encoding and readout directions are used as endspoilers in the pre-sequence SQ_(pre). Each gradient spoiler pulsedephases spins in each direction after a plurality of divided MT pulseshave been applied, excluding spin mutual interference from thepre-sequence and the data acquisition sequence. This is effective inpreventing occurrence of stimulated echoes. This spoiler pulse mightalternatively be applied only in any one or two directions.

The data acquisition sequence SQ_(acq) is executed as an FSE method, forexample, including a slice gradient G_(S), read-out direction G_(R) andphase-encoding direction G_(E).

The host computer 6 executes a given main program, during which time itapplies pulses in a pulse sequence such as shown in FIG. 3. Thisapplication of the pulses is done via the x, y and z-coils 3 x-3 z andthe RF coil 7, under control of sequencer 5.

To begin with, in the pre-sequence SQ_(pre), the divided n-piece MTpulses P₁-P_(n) (n is an integer larger than 1; for example, n=10) eachhaving a flip angle FA of α degrees (for example, α=90 degrees) areapplied in sequence with the slice gradient (=strength G_(S1)).

Suppose that a region to be diagnosed is the inferior limb, as shown inFIG. 4B. Appropriately setting the strength G_(S1) of the slice gradientG_(S) allows a pre-excitation slice S_(mt) having a given thickness tobe located in parallel and almost contiguously to the artery-inflowingside of a desired slice S_(ima) to be imaged. Hence, the MT pulsesdivided into n-pieces are applied in turn every interval Δt apart.

Alternatively, adjusting the strength, for example, of the slicegradient G_(S) may locate the pre-excitation slice S_(mt) to thevein-inflowing side, that is, the artery-outflowing side of the imagingslice S_(ima). Between the imaging slice S_(ima) and the pre-excitationslice S_(mt), there may be either a proper gap or no gap, depending onnecessity.

Then, in the pre-sequence SQ_(pre), the divided MT pulses are followedby the gradient spoiler pulses SP_(S), SP_(R) and SP_(E) applied in theslice, readout and phase-encoding directions, respectively.

Thus, at first, a plurality of divided MT pulses is applied to thepre-excitation slice S_(mt). Namely, it is repeatedly excited during ashorter duration a plurality of times by the MT pulses with a smallerflip angle. This application gives rise to excitation of spins residingwithin the pre-excitation slice S_(mt). This excitation becomesoff-resonance against the imaging slice S_(ima), thus providing theimaging slice S_(ima) unique MT effects according to the presentinvention, which will be described later. Spins which have remained inthe lateral magnetization after the application of the divided MT pulsesare then dephased sufficiently by the spoiler pulses SP_(S), SP_(R) andSP_(E).

After this, the data acquisition sequence SQ_(acq) follows, wherescanning is performed on the imaging slice S_(ima) using an FSE method,for example, under control of sequencer 5. Because the slice gradientG_(S) whose strength is G_(S2) (≠G_(S1)) is now being used, the imagingslice S_(ima) is set at a desired imaging location. A plurality ofechoes responding to a plurality of refocusing RF pulses is acquiredfrom the imaging slice S_(ima) via the RF coil 7 and sent to thereceiver 8R. A series of such processes is carried out for every time ofexcitation.

Echo data acquired from a subject (patient) P are processed into digitalforms, and sequentially stored into the arithmetic operation unit 10.This unit 10 is responsive to a reconstruction command from the hostcomputer 6, so that a set of echo data mapped in the two-dimensionalFourier space are two-dimensionally Fourier-converted to produce MRAimage data of the imaging slice S_(ima).

This MRA image has fewer artifacts and a remarkably improved degree ofimage contrast between inflowing blood/parenchyma in comparison withheretofore conventional MT pulses. This is thanks to use of a pluralityof divided MT pulses according to the present invention. To be specific,this is because the echo signals from the parenchyma (stationaryportion) within the imaging slice S_(ima) decrease owing to MT effects,while MT effects occurring in blood flow (arteries and/or veins)inflowing into the imaging slice S_(ima) are reduced. That is, aplurality of divided short-duration MT pulses shorten the apparent T1relaxation time of blood that is flowing and tumbling, leading toreduced MT effects. On the other hand, the parenchyma suffers a simplesum of a plurality of divided MT pulses, reducing the signals therefromcorrespondingly (i.e., in relation to the sum). Therefore, compared tothe conventional one large MT pulse technique, image contrast betweeninflowing blood and parenchyma shows a greater improvement.

This feature is explained in detail from a principle viewpoint.

Considered herein are differences in MT effects giving flowing ortumbling blood between a first case in which one RF pulse having a longduration is applied as the MT pulse and a second case in which aplurality of RF pulses, each having a relatively short duration, areapplied in sequence.

General factors that contribute to T1 relaxation time (T1) will be firstexplained. The time T1 changes depending on temperature, paramagneticcomponents, the size of molecules, their environments, viscosity andothers, and can be expressed by the following factors in moleculesconstituting components.

1/T1=1/T1(DD)+1/T1(SR)+1/T1(SC)+1/T1(CSA)+ . . .

In this expression, the first term T1(DD) represents the internucleardipole-dipole interaction. This interaction moves energy produced by RFexcitation to lattices, as pictorially shown in FIG. 5, thereby decouplespins A and B of molecules coupled so as to increase signals.

The second term T1(SR) represents the rotational move of spins.Molecules coupled move rotationally. Although depending on magnitudes ofmotion, molecules under rotational motion generate local magneticfields, thereby contributing to shortened T1.

The third term T1(SC) represents scaler coupling. When one atom coupledwith quadrupole (atoms satisfying the spin quantum number I≧1; forexample, O¹⁷=5/2, m_(n) ⁵⁵=5/2; as a reference, H¹=½), the quadrupolehas an inherent shorter relaxation time referred to as a quadrupolerelation. Spins coupled with the quadrupole are also reduced in their T1relaxation time.

And the fourth term T1 (CSA) is a term called chemical shift anisotropy,which represents changes in local fields on changes in electronicshielding effects. This also affects the T1 relaxation time.

Thus, the T1 relaxation time depends on various factors. The foregoingfactors are not all, but major ones to affect the T1 relaxation time.

Even in stationary blood, oxyhemoglobin and deoxyhemoglobin haveparamagnetic ions (primarily, iron I and iron II), thus producing localmagnetic fields to reduce the T1 relaxation time. In addition, in termsof the T2 relaxation time, the presence or absence of O₂ has influenceon veins (with oxygen; T2=120 msec) and arteries (less oxygen; T2=220msec).

FIG. 6 shows the dependency of T1, T2 and T1P on Tc (i.e., effectivecorrelation). In the graph, a position A shows Tc of hard solid, anotherposition B does that of soft solid, another position C does that ofliquid of a high viscosity, and another one does that of ordinary liquid(refer to T. C. Farrar and E. D. Becker, Pulse and Fourier TransformNMR, p. 98, Academic Press, 1971).

Tc, which is called effective correlation, is a factor representingmotion of molecules. To be specific, it represents the degree oftumbling (rotation and vibration) of fast-moving molecules. In FIG. 6,as proceeding to the right along the lateral axis, the degree of solidgets higher, so molecules move slower. On this relationship, it isunderstood that the T1 relaxation time differs between fast-moving bloodand stationary blood.

Moreover, the application of a plurality of divided MT pulses causesflowing blood to shorten its apparent T1 relaxation time.

MT effects are, as described before, a phenomenon by which, when theequilibrium between free water Hf and macromolecules Hr which areneighboring to each other and in dipole-dipole interaction, protons ofthe macromolecules are excited, by an RF pulse of which frequency isoff-resonance against free water, so that signal intensities of freewater that interferes with protons are affected. When taking themagnetization of free water as Hf and that of macromolecules as Hr, andconstants of reaction time are k₁ and k⁻¹, the relation of

${{Hf}\underset{k_{- 1}}{\overset{k_{1}}{leftharpoons}}{Hr}},\mspace{31mu} {{rate} = {K\frac{\lbrack{Hf}\rbrack}{\lbrack{Hr}\rbrack}}}$

is obtained. Here, the square brackets show concentration and K is areaction constant.

Therefore, when letting the T1 relaxation time of free water be T1f andthat of macromolecules be T1r,

${{R\; 1\; f} = \frac{1}{T\; 1\; f}},\mspace{31mu} {{R\; 1\; r} = \frac{1}{T\; 1\; r}}$

are obtained. Where M_(f(t)) is the magnetization of free water attime=t, M_(f(0)) is that of free water at time=0, M_(r(t)) is that ofmacromolecules at time=t, and M_(r(0)) is that macromolecules at time=0,

$\frac{M_{f{(t)}}}{t} = {{{( {M_{f{(0)}} - M_{f{(t)}}} ) \cdot R}\; 1\; f} - {M_{0{(t)}} \cdot k_{1}} + {M_{r{(t)}} \cdot k_{- 1}}}$

is obtained.

Magnetization M_(fSAT) realized when the MT pulses are applied isexpressed by

$M_{fsat} = \frac{M_{f{(0)}}}{1 + {{k_{1} \cdot T}\; 1\; f}}$

and its longitudinal relaxation time T_(1SAT) is expressed by

$\frac{1}{T_{1\; {SAT}}} = {{R\; 1\; f} + k_{1}}$

Therefore, the reaction constant k₁ is

$k_{1} = {\frac{1}{T_{1\; {SAT}}}( {1 - \frac{M_{fSAT}}{M_{f{(0)}}}} )}$

where T_(1SAT) is an apparent T1 (refer to Balaban, Magn. Reson.Quarterly, Vol. 8, No, 2, 1992).

When expressing the magnetization of protons of free water in movingblood as HfA, that of macromolecules in moving blood as HrA, that offree water in stationary parenchyma as HfB, and that of macromoleculesin stationary parenchyma as HrB, the magnetization behaves aspictorially shown in FIG. 7, in response to the application of the MTpulses according to the present invention.

When the first divided MT pulse P1 (whose flip angle is 100 degrees, forexample) is applied to the magnetization HfA and HrA which are inequilibrium within moving blood (FIG. 7A), magnetic spins(magnetization) are transferred from the macromolecule magnetization HrA(nuclear pool) to the free water magnetization HfA (nuclear pool). Tothe contrary, energy is transferred from the free water magnetizationHfA to the macromolecule magnetization HrA (see FIG. 7B). Because adivided MT pulse is applied, amounts of transferred spins and energy areboth small, thereby providing low MT effects.

Owing to the longitudinal relaxation that follows the excitation, themacromolecule magnetization HrA returns to its initial equilibriumstate. In the course of the return process, the T1 relaxation time T1 isapparently shortened due to such factors as the dipole-dipoleinteraction and the paramagnetic effects, speeding up the return processof the magnetic spins HrA.

Such divided MT pulses P₂, P₃, . . . , P_(n) are applied in successionto the first one P₁. The entire MT effects are apparently loweredbecause of their shortened T1 relaxation time (refer to FIGS. 7C and7D). Consequently, compared to stationary or almost motionless blood andparenchyma, MT effects occurring in flowing (moving) blood are smaller,thus echo signals from an imaging slice S_(ima) rise in intensity.

In contrast, MT effects occurring in both the free water magnetizationHfB and the macromolecule magnetization HrB of stationary or almostmotionless parenchyma can be expressed with the foregoing FIG. 1. Inother words, a plurality of divided MT pulses serves only the sum oftheir magnetization HfB and HrB. Thus, the parenchyma of the imagingslice S_(ima) has the equivalent MT effects to the conventional MTeffects, which reduces echo signal intensities, as seen in aconventional MT pulse.

Accordingly, the MRI system is able to utilize slice-selective dividedMT pulses to make a distinction between stationary or almost motionlessobjects and moving objects. Compared to the conventional one MT pulsewhose duration is long and whose flip angle is large, contrast betweenblood (blood flow) and parenchyma of an imaging slice can be improvedgreatly, providing a higher depiction performance of blood flow.Therefore, a higher quality of MRA images can be provided.

In this embodiment, owing to the fact that the MR contrast medium is notused to obtain MRA images, non-invasiveness is still kept. Compared toMRA imaging with contrast mediums, patients suffer fewer physical andmental burdens.

Concerning this embodiment, variations can be provided at least asfollows. In the pulse sequence shown in FIG. 3, a plurality of slicegradient pulses G_(S) are applied concurrently with a plurality ofdivided MT pulses. This pulse train may be changed, however, as shown inFIG. 8, wherein only a one slice-gradient pulse G_(S) is continuouslyapplied over the entire application period of a plurality of divided MTpulses. This manner can shorten a time necessary for applying the MTpulse train P_(MT), realizing a shorter imaging time.

Another variation for applying a plurality of divided MT pulses is shownin FIG. 9. According to this application, no gradient pulse is appliedin any of the slice, readout and phase-encoding directions; a pluralityof divided MT pulses are first applied alone, then gradient spoilerpulses are applied in any one or more of the slice, readout andphase-encoding directions. Thus, the divided MT pulses are applied in aslice-non-selective fashion, and effective in a wider area, not limitedto whether a region to be imaged is a slice or slab. In foregoing FIGS.8 and 9, gradient pulses including spoiler pulses in the readout andphase-encoding directions are omitted from being drawn.

Still, data acquisition sequences available to the MRI system are notlimited to the foregoing FSE sequence, but may include other types ofpulse sequences based on various methods, such as FE, SE, EPI, FLAIR, orFASE methods.

Second Embodiment

Referring to FIGS. 10-17, a second embodiment of the present inventionwill be described.

In this embodiment, using a plurality of divided MT pulses describedabove, the parenchyma of the lungs of a patient will be imaged.

For MR-imaging the lungs, three approaches have been known primarily,which are to use a hyper-polarized gas (e.g., xenon or helium), toperform a perfusion imaging using a contrast medium Gd-DTPA (refer toHatabu H., et al., MRM 36:503-508, 1996), and to perform imaging withsuction of oxygen using oxygen molecules (refer to Edelman R. R., etal., Nature Medicine 2, 11, pp. 1236-1239, 1996).

Of these, the first approach is based on imaging at the MR frequency of,for example, a xenon gas (Xe) suctioned into the lungs. The second oneis a technique to observe a perfused state of Gd-DTPA in blood. Thethird one utilizes a report that oxygen molecules, which are weaklyparamagnetic, cause a signal from water to change sufficiently at thesurface of the pulmonary alveolus, the water signal being observable byMRI.

However, the first approach needs an ordinary xenon gas to behyper-polarized, leading to a high cost for producing the gas. Thesecond one that uses an MR contrast medium for perfusion imagingrequires an invasive treatment against patients. Mental and physicalburdens of patients are enormous, in addition to a high examinationcost. In some cases, a patient's specific character shows the rejectionagainst MR contrast medium. Moreover, the third one with the suction ofoxygen molecules is difficult to gain sufficient signal changes in animage, no providing images as satisfactory as it could be used forresearch or distinct contour information.

The lungs are constructed such that most of their surfaces are occupiedby the spongy pulmonary alveolus, bronchus, pulmonary artery, andpulmonary vein surrounded by air. The surface of the spongy pulmonaryalveolus amounts to a huge area, but it does not have free water insideand outside the cells, unlike other organs (such as the liver and renalgland). Thus, in the case of the lungs, the signal of water, which is anobjective to be detected in MRI, is only detected from their bloodsystems, so the water signal from the parenchyma thereof is absolutelyshort. Therefore, it is considered that a region surrounding thepulmonary alveolus is difficult to be reflected into MR signals, becauseof shortage of water molecules compared to an amount of their surfaces,although the lungs have a T2 value of 80 msec comparable to other organs(refer to JMRI, 2(S):13-17, 1992). As a result, conventional MR imagingwas required to use MR contrast mediums.

This embodiment utilizes fully the advantages of a plurality of dividedMT pulses so that the parenchyma of the lungs, which has been difficultto be imaged with no contrast medium, is imaged.

An MRI system of this embodiment is configured similarly to that of thefirst embodiment, but different from the first one in scan procedures asbelow.

Subsequent to preparation work including a not-shown positioning scanand the input of imaging conditions, the host computer 6 performsimaging at least twice. Such imaging is performed two times, forexample, as shown in FIG. 10, in each of which is a two- orthree-dimensional scan that acquires a set of echo data necessary forimage reconstruction. It is preferred that each scan be performed with apatient's breath hold and an ECG gating technique.

A pulse sequence available for this imaging may be a two-dimensional orthree-dimensional scan, but should be based on a Fourier transformmethod. Its pulse train may use an SE, FSE (Fast SE), FASE (FastAsymmetric SE), FE, FFE(Fast FE), segmented FFE, EPI (Echo PlanarImaging), and other methods.

In addition to the reconstruction processing of raw data, the arithmeticoperation unit 10 can perform synthesis processing and differenceprocessing of image data. Such synthesis processing includes addition ofa plurality of frames of image data pixel-by-pixel, maximum intensityprojection (MIP) for selecting maximums along rays through a set ofthree-dimensional image data, and others. Alternatively, the image datasynthesis processing can be performed by adding as-acquired raw data toeach other with the axes of a plurality of frames matched with eachother in the Fourier space. In addition, the addition includes simpleaddition, averaging, weighted addition, and others.

The storage unit 11 is able to store not only reconstructed image data,but also synthesized (or difference) image data. The input device 13 isused to provide the host computer 6 desired imaging conditions, the typeof pulse sequence, information notifying the image synthesis processingor difference processing.

The operation of the MRI system will now be described with reference toFIGS. 10-17.

As shown in FIG. 10, the host computer 6 performs imaging two timesusing a three-dimensional (3D) FASE method employed as one example. Thefirst imaging is carried out with breath hold and ECG-gating techniques,but with no application of a plurality of divided MT pulses (MTpulses=off). The second imaging starts at an appropriate moment afterthe first imaging terminated. Like the first imaging, the second imaginguses breath hold and ECG-gating techniques, in addition to applicationof a plurality of divided MT pulses (MT pulses=on).

FIG. 13 exemplifies a pulse sequence used in both the first and secondimaging scans. The pulse sequence is based on a three-dimensional FASEmethod (Fast Asymmetric SE method made by an FSE method with ahalf-Fourier technique). In FIG. 13, the phase-encoding gradientsincluding a spoiler pulse are not shown.

In the case of the first imaging, a data acquisition sequence SQ_(acq)shown in FIG. 13 is only applied, while a pre-sequence SQ_(pre) is notused, though FIG. 13 shows the pre-sequence (FIG. 13 shows both thecases for the first and second imaging). That is, an MT pulse trainP_(MT) and gradient spoiler pulses SP_(S), SP_(R) and SP_(E) thatcompose the pre-sequence SQ_(pre) are not applied in the first imaging.Scanning in the first imaging is performed without the MT pulses (off).

In contrast, for the second imaging, as shown in FIG. 13, prior to thedata acquisition sequence SQ_(acq), the pre-sequence SQ_(pre) isapplied. Thus, a plurality of divided MT pulses is applied that areformed in the same way as in the first embodiment.

Alternatively, although not shown in FIG. 13, the pulse sequence used inthe second imaging may be constructed such that, with the sliceselection for the RF pulses of the data acquisition sequence SQ_(acq) isunchanged, gradients for selecting a region to which the divided MTpulses are applied are also applied to the phase-encoding and readoutdirections, in addition to the slice direction.

As one example, in the three-dimensional FASE pulse sequence shown inFIG. 13, the effective echo time TE_(eff) and the echo train spacing areset to 100 msec and 5 msec, respectively. As to the divided plural MTpulses, the frequency offset of 1300 Hz and five divided MT pulses areset (a total flip angle of the five divided MT pulses is 800 degrees,for example). Further, the repetition time TR is 3247 msec, the flipangles of flip/flop pulses are 90/140 degrees, the matrix size is256×256, and the FOV (field of view) is 37 cm×37 cm. When consideringthe fact that the vessels of the lungs run in all the directionstherein, it is preferred that scanning for each imaging is performed aplurality of times as the phase-encoding direction is changed each time,and data acquired each time are subject to their averagespixel-by-pixel. This technique (called SPEED technique) of combiningswapping the phase-encoding direction and averaging has been disclosedby J. of Magn. Reson. Imaging (JMRI) 8:503-507, 1998. For instance,scanning is performed two times with the phase-encoding directionaltered by 90 degrees and data acquired each time are averaged toproduce image data in each time of imaging.

First, the first imaging is carried out as below. The host computer 6executes the processing shown in FIG. 11 in response to operativeinformation from the input device 13.

Specifically, the host computer 6 reads from the input device 13 anoptimum ECG-gating delay time T_(DL) appropriately determined orselected by the operator (step S20).

Then, the host computer 6 inputs scan conditions (for example, thedirection of phase encode, an image size, the number of times of scans,a standby time between scans, and a pulse sequence dependent on a regionto be scanned, and others) and information about image processing(addition, MIP, or others), converts those bits of information intocontrol data, and outputs the control data to sequencer 5 and arithmeticoperation unit 10 (step S21).

If it is then judged that an instruction indicating the completion ofpreparations has been issued (step S22), a command indicating the startof a breath hold is output to the voice generator 16 (step S23). Thiscauses the voice generator 16 to utter a voice message saying, “Holdyour breath, please.” In response to this message, the patient holds heror his breathing.

After commanding the start of the breath hold, host computer 6 orderssequencer 5 to start imaging (step S24; refer to FIG. 12).

As shown FIG. 12, when receiving the command for starting imaging (stepS24-1), sequencer 5 starts to read an ECG signal (step S24-2), anddetermines a predetermined n-th R-wave peak (reference wave) thatappears in the ECG signal by observing an ECG triggering signalsynchronous with the peak (step S24-3). The reason the appearance of then-th R-wave (for example, n=2) is waited is to obtain a steady state ofthe breath holding. This waiting technique produces an adjusting timeT_(sp) shown in FIG. 13.

When the n-th R-wave peak appears, the processing is brought into awaiting period corresponding to the predetermined delay time T_(DL)(step S24-4). The delay time T_(DL) has been optimized such that theecho signal strength is the highest for imaging the pulmonary tissue,and its depiction performance is superior.

As it is determined that a time instant when the delay time T_(DL)elapsed corresponds to an optimum ECG-gating time, sequencer 5 startsscanning (step S24-5). Specifically, sequencer 5 drives transmitter 8Tand gradient power supply 4 according to pulse-sequence information thathas already been transmitted and stored, and executes the first scanningon the three-dimensional FASE-based pulse sequence with the ECG gating,as shown in FIG. 13. In this scanning, the MT pulse train P_(MT) is notapplied in the pre-sequence SQ_(pre) (that is, MT pulse=off). By thisscanning, echo signals are acquired from a three-dimensional regioncontaining the lungs, as illustrated in FIG. 14, for an interval ofabout 600 msec under an amount SE1 of the first slice encoding.

After the completion of the first scanning under the one slice-encodingamount SE₁, sequencer 5 determines whether scanning under the finalslice-encoding amount SE_(n) was finished or not (step S24-6). If thedetermination is NO at this step, the processing undergoes a waitingprocess for a relatively shorter interval (for example, 2 heartbeats;2R-R) from the R-wave used for the scanning as monitoring the ECG signal(step S24-7). This shorter waiting process deliberately suppresses therelaxation of the longitudinal magnetization of spins in the stationaryparenchyma. This waiting time determines the repetition time TR.

In response to the appearance of the third R-wave (YES at step S24-7),sequencer 5 returns processing to the foregoing step S24-4. Therefore,scanning is performed in the same way as above, under the secondslice-encoding amount SE₂, from a time instant when the specified delaytime T_(DL) passed from a ECG triggering signal representing the thirdR-wave peak (steps S24-4, 5). This scanning also allows the acquisitionof echo signals from the three-dimensional region R_(ima). Likewise,such scanning is repeated to acquire echo signals until the finalslice-encoding amount SE_(n) (for example, n=8).

When the final scanning has been finished, the determination performedat step S24-6 becomes YES, sequencer 5 informs host computer 6 of thecompletion of the scanning (step S24-8). Thus, the processing isreturned to host computer 6.

The host computer 6, when receiving the completion of scanning data fromsequencer 5 (step S25; FIG. 11), outputs a command of release of breathhold to the voice generator 16 (step S26). The voice generator 16 thenutters a voice message toward the patient saying, for example, “You canbreathe.” This will terminate the interval of breath holding.

Therefore, as illustrated by FIG. 13 sequence, on the basis of theECG-gating technique of every 2R-R, the scanning is performed n-times(for example, n=8) for each slice-encoding amount. A duration requiredfor these n-time scans, that is, an interval that a patient is requiredto continue breath holding, is 20-25 seconds, as one example, althoughthis interval depends on imaging conditions.

Spin echo signals produced in the patient P for each scan are receivedby the RF coil 7, and sent to the receiver 8R. The receiver 8R performsvarious kinds of pre-processing with the spin echo signals. The signalsare thus converted into digital quantities. The digital echo data aresent to the arithmetic operation unit 10 via sequencer 5 and mapped in athree-dimensional image k-space formed in its incorporated memories.Because this embodiment is based on the half-Fourier method, data in thek-space that are not acquired are computed using the already acquireddata, and additionally mapped in, providing the full echo data in thewhole k-space.

When a predetermined standby time passes after the first imaging, thesecond imaging is performed in the similar manner to that described byFIGS. 11 and 12. Only a difference from the first one is that, at stepS21 in FIG. 11, information about applying the MT pulse train P_(MT) (MTpulses=on) in the pre-sequence SQ_(pre) is provided to the host computer6, and such control data as scan conditions including the information,image processing method and others given to sequencer 5. The otherscanning conditions are the same as those in the first imaging. Thus,the pulse sequence on the three-dimensional FASE method performed atstep S24-5 in FIG. 12 includes a pre-sequence SQ_(pre) and a dataacquisition sequence SQ_(acq), as shown in FIG. 13. Namely, a pluralityof divided MT pulses is added at the front of each scan.

Hence, echo data acquired by this second imaging are also mapped in theimage k-data in the same way as that in the first imaging.

When the data acquisition processing is completed in such way for thetwo times of imaging, host computer 6 commands arithmetic operation unit10 to process and display image data. A series of these processes areshown in FIG. 15.

The arithmetic operation unit 10 performs a three-dimensional transformwith echo data in k-space acquired and mapped by the first imaging scan,thereby reconstructing absolute-value image data IM1 in real space (FIG.15, step 31). Likewise, for those acquired and mapped by the secondimaging scan, the same reconstruction as above is performed to provideabsolute-value image data IM2 reconstructed into real space (step 32).

FIGS. 16A and 16B pictorially show signal levels of image data inpulmonary axial images produced on this embodiment (for sake ofconvenience, the both are shown as two-dimensional images). FIG. 16Ashows an image IM1 when the MT pulses are turned off, while FIG. 16Bshows another image IM2 when the MT pulses are turned on. In the case ofFIG. 16B, due to the MT effects accompanied with the application of thedivided MT pulses, the signal values are lowered, but when such loweringis compared between the parenchyma and the blood, the parenchyma isgreater in degrees of such lowering than the blood. In FIG. 16B, onlynarrow hatching lines pictorially show the parenchyma portion where thedegrees of lowering are greater.

More detailed explanation about such comparison will be given usingFIGS. 17A and 17B. Because the MT pulses are turned off for the firstimage IM1, the signal levels in the tissue and blood of the lungs LG aredetermined into certain values depending on imaging conditions. Thesecond image data IM2 are, however, obtained with the divided MT pulses.Therefore, the parenchyma of the lungs LG has larger MT effects thanthat occurring in the blood. Thus, as illustrated in FIG. 17B, a ratioΔT between reduced signal levels for the parenchyma is larger that ΔBfor the blood. This difference “ΔT−ΔB” between the ratios ΔT and ΔBcontributes image data of the parenchyma of the lungs LG.

Hence, using a properly selected coefficient α (0<α≦1), the arithmeticoperation unit 10 performs pixel-by-pixel a difference operation of“IM1-α·IM2” with both the absolute-value image data IM1 and IM2 (FIG.15, step S33). The resultant image data IM3 are illustrated in FIG. 16C(for sake of convenience, illustrated by a two-dimensional image). Asshown therein, the signals in the blood of the lungs LG are almostcancelled out due to the difference computation, leaving only theparenchyma in the image IM3.

Then, a maximum intensity projection (MIP) process is done withthus-produced three-dimensional real-space image data IM3, producing atwo-dimensional image data (FIG. 15, step S34). Such image data are notmerely displayed by the display unit 12 but also stored in the storageunit 11. The three-dimensional image data IM3 are stored therein as well(step S35).

As described above, since a plurality of divided MT pulses used by theMRI system of this embodiment operate just as the sum of MT pulses forthe stationary or almost stationary lung parenchyma, giving it larger MTeffects. Compared with blood flow in the lungs, the signal value oftheir parenchyma is reduced largely. Thus, difference processing (simpleor weighting difference processing) with an image obtained with no MTpulse can image the parenchyma of the lungs. Although it was thoughtbefore that imaging the parenchyma of the lungs without using gases,contrast mediums, or oxygen was impossible, it can preferably be imagedby the present embodiment.

Particularly, there is no need for injecting contrast mediums intopatients, so non-invasive imaging can be done and mental and physicalburdens given patients are remarkably relieved. Concurrently with it, anoperator is free from cumbersome operations inherent to the contrastmedium method, such as it is necessary to measure a time for bestobtaining contrast effects. Moreover, differently from the contrastmedium method, imaging can be repeatedly performed easily by thisembodiment, if necessary. In terms of imaging cost, this embodiment isadvantageous because a high-cost contrast medium or gas is not used.

In the present embodiment, the gradient spoiler pulses are applied onlyone time at the last after a plurality of MT pulses last in turn. Thisapplication enables less (or almost zero) standby time between twoneighboring MT pulses. Thus, the longitudinal relaxation ofmagnetization between two MT pulses is kept to a small quantity, therebyproducing larger signal values.

Furthermore, the repetition time TR and the train spacing can be set toshorter lengths and the slice direction can be set in the front/backdirection of a patient. Thus, the entire scan time can be reduced. Thenumber of times of slice encodes may be less because of a shorterimaging length in the slice direction. The whole imaging time isshortened greatly compared to the conventional TOF or phase contrastmethod. A patient throughput is increased.

In addition, scanning for each of two times of imaging (scanning forobtaining a set of objective echo data) can be accomplished within onetime of possible breath holding, relieving a burden on a patient.Further, suppressed are motion artifacts due to cyclic motions of thelungs and the like or due to the shifts of the body itself over aplurality of times of breath holding scans. Thus, less-artifact imagescan be provided.

Moreover, the use of the ECG gating can provide images from which motionartifacts caused by the motions of the heart are almost adequatelyexcluded.

By the way, according to the ECG gating according to this embodiment,scanning is constructed so as to begin at a time phase delayed from anR-wave by a certain delay time T_(DL). Alternatively, the time phase tostart scanning may be set to other time phases depending on individualclinical demands.

Third Embodiment

Referring to FIGS. 18-20, a third embodiment of the present inventionwill be described.

The hardware construction of an MRI system of this embodiment is similarto those described in the foregoing embodiments.

The basic feature of the MRI system according to this embodiment is touse pulse sequences based on a single-shot (RF excitation) fast SE (spinecho) method on three-dimensional or two-dimensional slab (thick slice)scanning under ECG gating, in which MT (magnetization transfer) pulsesof which flip angles and of which number are changeable are applied. TheMT pulses are applied to give images MT contrasts based on MT effectsfitted to imaging objects, and functions as means for producing aneffective contrast between tissue and blood. Such pulse sequences aremost effective in depicting the cardiac blood vessel systems. Forinstance, abdomen organs such as the heart can be imaged to provideT2-weighted contrast images. These images provide tissue contrastbetween the cardiac muscle and the blood as high as that obtained by theconventional FE-system pulse sequences, as well as a higher spatialresolution.

To realize such imaging, in the MRI system of this embodiment, hostcomputer 6 and sequencer 5, shown in FIGS. 18 and 19, operate togetherto perform a scan on a pulse sequence preferred for imaging the cardiacblood systems. This pulse sequence is composed of a pre-sequenceSQ_(pre) performed first and an imaging data acquisition sequenceSQ_(acq) that follows the pre-sequence. The pulse sequence is executedwith computer control by sequencer 5 instructed from host computer 6that is responsible for a predetermined main program not shown. MRsignals acquired with this scanning are processed under a predeterminedreconstruction routine by arithmetic operation unit 10. Moreover, a MIP(maximum intensity projection) image for showing the blood vesselsystems can be formed by arithmetic operation unit 10.

Sequencer 5, which has a CPU and memories, stores pulse-sequenceinformation sent from host computer 6, and controls a series ofoperations to be performed by gradient power supply 4, transmitter 8Tand receiver 8R according to the stored information.

What is referred to pulse-sequence information is all informationrequired for operating gradient power supply 4, transmitter 8T andreceiver 8R according to a pulse sequence.

For example, pulse-sequence information includes information concerningthe strength of a pulsating current to be applied to the x-, y- andz-coils 3 x-3 z, and the application time and timing thereof.Additionally, sequencer 5 receives the digital echo data from receiver8R and transfers them to arithmetic operation unit 10.

As for the data acquisition sequence SQ_(acq) (shown in FIGS. 18 and 19)forming part of the pulse sequence, a two-dimensional (2D) slab scan ora three-dimensional (3D) scan into which a Fourier transform method isincorporated can be adopted, for example. Available fundamental pulsetrains are SE-system pulse trains in which one time of spin excitationcan produce a plurality of echo signals. Such pulse trains includevarious ones, such as a fast SE method (2D or 3D), an FASE (fastasymmetric spin echo) (2D or 3D) combining a fast SE method with ahalf-Fourier technique by which data acquisition for one slice or oneslice encode is carried out for one time or a plurality of times of RFexcitation (shot), an EPI (echo planar imaging) method categorized intothe SE system, or a hybrid EPI method.

The arithmetic operation unit 10 receives digital echo data sent fromthe receiver 8R via sequencer 5, maps the data in a Fourier space (orthe k-space or frequency space) formed in an incorporated memory, andperforms a two-dimensional or three-dimensional Fourier transform withthe mapped data so as to reconstruct an image in the real space.Moreover, arithmetic operation unit 10 carries out mutual synthesis ofreconstructed image data. The Fourier transform may be performed by hostcomputer 6, not by sequencer 5.

A preferred example of synthesis is addition in which reconstructedimage data items of a plurality of frames are added up pixel-by-pixel ormaximum intensity projection (MIP) in which a maximum pixel value isselected pixel-by-pixel from among reconstructed image data items of aplurality of frames. Addition includes simple addition, averaging, andweighting and addition.

The voice generator 16, which consists of a constituent of the breathhold instructing components, utters, for example, a voice messageinforming a patient of the start or end of a breath hold in response toa command sent from host computer 6. In place of this voice generator,provided is a light generator instructing a patient of timing of breathhold with light on/off signals. Alternatively or in parallel with thisvoice massage, this embodiment uses another technique to inform thebreath hold; that is, it is a method to use sounds generated by a gantrywhen gradient pulses are applied in a controlled manner.

A pulse sequence performed in this embodiment will be described withreference to FIGS. 18 and 19.

As shown in FIG. 18, the pulse sequence consists of a pulse train withan ECG-gating technique by which a data acquisition sequence is startedwith a delay of a predetermined time from a triggering signal insynchronism with R-waves of an ECG signal. The data acquisition sequenceserves as a main scan for acquiring MR signals. Practically, the pulsesequence includes a pre-sequence SQ_(pre) executed prior to the mainscan and a data acquisition sequence SQ_(acq) (main scan) that followsthe pre-sequence. The delay time is set so that the data acquisitionsequence SQ_(acq) is carried out for an optimum period within oneheartbeat.

The pre-sequence SQ_(pre) includes an MT pulse train PT_(mt) that causesMT effects, a CHESS (chemical shift selective) pulse P_(chess) thatsuppresses the acquisition of MR signals emanated from fat (called fatsuppression), gradient spoiler pulses SP_(s), SP_(r) and SP_(e) fordephasing spins.

The MT pulse train PT_(mt), which is shown in detail in FIG. 19, has aplurality of divided MT pulses MT₁-MT_(n) and a plurality of slicegradient pulses G_(S) each applied in parallel with the pulsesMT₁-MT_(n). Pluralities of MT pulses MT₁-MT_(n), which compose a trainof multiple MT pulses, are continuously applied with no gaptherebetween. A minute standby time Δt may be set between two MT pulses.

Each slice gradient pulse G_(S) is turned on/off with a gapless state tothe next one, concurrently with MT pulses MT₁-MT_(n). As an alternativeconstruction, pluralities of slice gradient pulses G_(S) may be added inthe form of a continuous one pulse. The polarity of this slice gradientpulse G_(S) is set, as one example, such that, as shown in FIG. 20, anexcitation region R_(ex) opposite to a flow of blood B to be imagedinflowing into an imaging volume region R_(ima) is excited withoff-resonance. This off-resonance excitation is done to expect theinflow of fresh blood flow B, without previously being excited, into animaging volume region R_(ima).

Each MT pulse is an excitation RF pulse formed, for instance, bymodulating with a sin c function an RF signal of a desired frequencyoffset value. The number of MT pulses are plural (for example, ten).Thus, the MT pulses compose a plurality of divided MT pulses describedbefore. The number of MT pulses, the value of a flip angle and thenumber of an off-resonance frequency is properly altered by autocalculation with computers or manual calculation.

After n-sets of the MT pulse and the slice gradient pulse are applied inturn, the CHESS pulse P_(chess) for fat suppression is applied. TheCHESS pulse P_(chess) is an RF pulse formed by modulating a desired RFsignal with a sin c function whose τ-length corresponds to the chemicalsifts between the proton spins of water and fat.

Following the application of the CHESS pulse P_(chess), thespin-dephasing gradient spoiler pulses SP_(s), SP_(p) and SP_(r) areapplied only one time at a time in the slice, phase-encoding and readoutdirections.

An objective to be imaged in this embodiment is a blood vessel system ofthe chest. In order to image this blood vessel system, it is necessarythat both an ECG-gating technique and a T2-weighting imaging techniquethat requires the repetition time TR to be more than 1000 msec be usedtogether and the ECG gating be performed over at least two heartbeats.Thus, to prevent the imaging from being longer to increase an efficiencyof imaging, the data acquisition sequence SQ_(acq) is composed of asingle-shot FSE method employing a half-Fourier technique on the basisof a three-dimensional or two-dimensional slab scan. The two-dimensionalslab scan is a scan performed both in a slice thickness of about 30-60mm and in a state that the number of multislices is 1-3 (that is, thenumber of slices is a few and MT effects occurring in them areequivalent to one slice).

Additionally, in the single-shot FSE method, an interval betweenmutually adjoining refocusing RF pulses, i.e., an ETS (echo trainspacing) between mutually adjoining echo signals is shortened to about 5msec.

The data acquisition SE sequence may have refocus RF pulses betweenwhich a time interval is set to a value of not more than 6 msec.

The operation of this embodiment will now be described.

When the MRI system is activated, host computer 6 and sequencer 5perform the pulse sequence shown in FIG. 18. The application of pulsesaccording to this sequence is carried out via the x-, y- and z-coils 3x-3 z and the RF coil 7 under the control of sequencer 5.

Host computer 6 and sequencer 5 monitor the input of a triggering pulsefrom the ECG signal. In response to the triggering pulse which has beeninputted, sequencer 5 waits for an appropriately predetermined delaytime T_(D) (for example, 300-500 msec) enabling the data acquisitionsequence SQ_(acq) to be delayed properly. The delay time T_(D) is set toa proper value in advance so that imaging is allowed in an optimum rangeof one heartbeat, where flow of blood outputted from the heart is stablein a mesodiastole.

On completing the above weighting period, sequencer 5 orders theperformance of the pulse sequence shown in FIGS. 18 and 19. Concurrentlytherewith, host computer 6 informs a patient of her or his breath holdvia voice generator 14 in response to the triggering pulse in a mannersuch that the breath hold fully covers the execution period of the pulsesequence.

When the pulse sequence is initiated, first, a plurality of divided MTpulses MT₁-MT_(n) composing the MT pulse train PT_(mt) and the slicegradient pulses G_(S) each having a predetermined strength and polarityare applied to an excitation region R_(ex) in sequence in a sliceselective mode. Each MT pulse has a frequency band that offsets by apredetermined value against a region R_(ima) to be imaged. Inconsequence, the divided plural MT pulses give off-resonance excitationto the imaging volume region R_(ima). Thus, the spins of the parenchymaand blood existing in the imaging volume region R_(ima) are subject togradually divided MT effects.

As mentioned before, these MT effects function as the sum of each MTpulse for the stationary parenchyma of the imaging volume regionR_(ima), the entire MT effects being almost equal to a state where asingle MT pulse having a large flip angle (for example, 1000 degrees) isapplied. Namely the MT effects are large, and echo signals lateracquired show a substantial decrease in intensity. On the contrary,blood flowing through the imaging volume region R_(ima) receives largelyweakened MT effects as a whole, because of the divided MT pulses. Echosignals from such blood, therefore, do not decrease so much inintensity. Additionally, as shown in FIG. 20, a blood B inflowing intothe opposite side of the imaging volume region R_(ima) to the excitationregion R_(ex) is still fresh, because MT effects are scarcely giventhem.

After the application of the MT pulse train PT_(mt), sequencer 5instructs necessary components to apply the CHESS pulse P_(chess). Inresponse to this, the CHESS pulse is applied to a region containing theimaging volume region R_(ima) for fat suppression. Only the protons offat resonate with the CHESS pulse, resulting in previous saturation ofspins.

In succession, gradient spoiler pulses SP_(s), SP_(r) and SP_(e) areapplied as end spoilers in the slice, readout and phase-encodingdirections. Thus, spins still left in the lateral magnetization aresufficiently dephased in each direction and saturated by spoilersSP_(s), SP_(r) and SP_(e). By this application, signals from the fatprotons become zero effectively, excluding interference of spin phaseswith the data acquisition sequence, and preventing occurrence of pseudoechoes. The spoiler pulse may be applied in only any one or twodirections.

At a time when the pre-sequence SQ_(pre) ends, the above-said delay timeT_(D) is to elapse. Sequencer 5 immediately starts performing the dataacquisition sequence SQ_(acq). As stated before, this sequence SQ_(acq)is performed based on a 3D single-shot FSE with a half-Fouriertechnique. With this FSE sequence, under each amount G_(E2) of sliceencoding obtained by altering the slice gradient G_(S), a plurality ofecho signals responding to a plurality of refocusing RF pulses areacquired. Namely, echo signals that correspond to one slice-encodingamount are acquired by one shot.

The “3D single shot” referred to herein is used to mean both (i) alldata necessary for reconstructing a 3D image are acquired at one time ofexcitation and (ii) all data corresponding to one slice-encoding amountare acquired by one time of excitation. (However, since the half-Fouriertechnique is adopted in this embodiment, “all data” does not mean allphase-encoded data corresponding to one slice-encoding amount.)

Echo signals for one slice-encoding amount are sequentially sent toreceiver 8R via RF coil 7. The echo signals are processed into digitaldata by the receiver 8R, then stored into arithmetic operation unit 10.

The acquisition of all the echo data that correspond to oneslice-encoding amount is completed within a period of about 300-500msec. It is preferred that the pulse sequence for one slice encode becompleted within one heartbeat, as illustrated in FIG. 18.

After the first scanning, sequencer 5 waits for a plurality ofheartbeats without performing any scan. For example, this waiting lastsuntil the triggering pulse associated with the third heartbeat isinputted. When such triggering pulse is received, sequencer 5 performsthe same pulse sequencer as that described above and host computer 6concurrently commands the breath hold. As a result, the repetition timeTR is kept to, for example, a period of 3R-R, which normally takesapprox. 3000 msec. It is, therefore, possible to provide T2-weightingimages.

By performing the ECG-gating imaging a plurality of times responsivelyto the R-waves appearing, for example, every 3R-R, three-dimensionalimage data are acquired from the imaging volume region R_(ima). Theacquired data are then mapped in a three-dimensional Fourier space setin memories incorporated in unit 10. On completing data acquisition,arithmetic operation unit 10 receives a command for image reconstructionfrom host computer 6. Responsively to this command, unit 10 performs athree-dimensional Fourier transform to reconstruct the mapped echo datainto image data in the real space. In addition, arithmetic operationunit 10 performs MIP processing with the reconstructed three-dimensionalimage data so as to produce a two-dimensional MR image. Thesethree-dimensional and two-dimensional data may be stored in storage unit11 and presented on display unit 12.

MR images of, for example, the blood system in the chest obtained by theMRI system of the present embodiment have a wide variety of advantagesand features as below.

(1) Firstly, the pulse sequence based on the FSE method with thehalf-Fourier technique (i.e., FASE method) is performed with theECG-gating technique and the repetition time TR of more than 1000 msecis secured. In consequence, T2-weighted contrast images of clinicallysignificant is surely obtained.

(2) In obtaining such T2-weighted images, scanning is done at one shot.Therefore, although it is assured that the repetition time TR has asufficient interval, an imaging time can be avoided from being longer.

(3) In this embodiment, since the data acquisition for T2-weightedimages is done in relatively longer intervals of ECG gating, a periodfor the data acquisition sequence in the repetition time TR is shortenedsignificantly. Because of this, an intermittent breath hold can beinformed with adequate control means. Using both the ECG-gating andbreath-holding techniques together can avoid artifacts owing to therespiratory move of the heart from occurring in a steady fashion.

(4) Further, in addition to one shot imaging in the 3D scan for oneslice-encoding amount, the echo train spacing (ETS) is shortened and theexcitation slab is thickened. This manner can not only suppress anincrease in blood signal but also reduce motion artifacts.

(5) Furthermore, the MT pulse is used as a plurality of divided MTpulses. Therefore, as stated before, a contrast between stationaryparenchyma and flowing or tumbling blood can be provided highly.

(6) Because the 3D scan (including the 2D scan of the less number ofslices or single slice) is adopted, it is not required to performmultislice imaging. Hence, the number of times of RF excitation can bereduced, avoiding a noticeable decrease in signal intensity, whichinherently occurs due to MT effects when the cardiac muscle tissue, theliver parenchyma, or others are multislice-imaged with the FSE method.It is known that MT effects are effective in depicting flow of blood byimproving a tissue contrast of the heart. In this embodiment, as aplurality of divided MT pulses is gradually applied, the total amount ofMT effects can be controlled, thus providing an appropriate andeffective tissue contrast.

(7) Where pulse sequences according to the FE system are used,shortening the entire data acquisition time necessitates the repetitiontime TR to be shortened. In order to meet such demand, a segmentedtechnique based on the ECG gating is carried out every gate. Incontrast, a T2-weighted imaging in compliance with the FSE method of thepresent embodiment permits the repetition time TR to be longer, withoutan extreme prolonged scan time. Thus, a standby time (for example,300-600 msec) from the triggering pulse responding to an R-wave to thediastole suitable for heart imaging can effectively be used to instructthe breath hold and apply the MT pulses and CHESS pulse. It is clearthat there is no temporal limitation on performing the pre-sequenceSQ_(pre). This eliminates the need for increasing a maximum of SAR (RFexposure) or maximum amplitudes of RF pulses. The number of MT pulses,their flip angles, and others can be set independently on each other toimprove an image contrast with a high degree of freedom in setting thefactors. A desired amount of MT effects is obtained at a minimum SARvalue.

(8) The gradient spoiler pulses are applied only one time at the last ofa train of pulses including the MT pulse train and CHESS pulse, which isable to shorten the entire pre-sequence time. So, a plurality of dividedMT pulses each having a shortened duration is capable of generatingsteady MT effects.

(9) In applying the MT pulses, the polarities of the slice gradientpulses are controlled to excite in an off-resonance manner the oppositeside of an excitation region R_(ex) to inflowing blood. Thus, the spinswithin the imaging volume region R_(ima) are not excited by the MTpulses, but undergo only MT effects. Desired flow of blood inflows intothe imaging volume region R_(ima) in a state that their spins have notbeen excited yet. As a result, signal intensities from the blood flowbecome high and a contrast between parenchyma and blood flow isimproved.

(10) Moreover, intervals between the refocus RF pulses are shortened,which prevents the drops of signals from blood or the occurrence ofmotion artifacts which were problems against the conventional FSEmethod. In association with such shortened intervals, there are otheradvantages:

-   -   a) the windows for data acquisition can be shortened, leading to        a reduced influence on the motions of organs or the body, which        reduces the occurrence of artifacts,    -   b) an amount of blood outflowing for an interval between        adjoining refocus RF pulses can be lowered, increasing signal        intensities from blood,    -   c) blurring of images resulting from T2 relaxation can be        improved, improving an image resolution, and    -   d) changes in the echo time TE in the central region in the        phase-encoding of the k-space can be kept low, providing an        excellent degree of image contrast equivalent or near to a        degree of image contrast obtained by the SE scan of which echo        time TE is the same.

(11) Still, the MRI system of this embodiment is able to provide anoptimum echo train spacing ETS in the three-dimensional scan. In thetwo-dimensional scan, a fact that shortened intervals between theadjoining refocus RF pulses raise a depiction performance of bloodsystems has been known. However, findings through experiment and othersconducted by the inventors show that simply applying into thethree-dimensional scan the echo train spacing which has been used in thetwo-dimensional scan hardly depicts blood systems. It was also foundthat in the three-dimensional imaging, the length of the echo trainspacing is very sensitive in depicting the blood systems of the chest.

In particular, even if an echo time spacing (ETS) of 10-15 msec whichhas been used by the conventional FSE scan is applied to imaging theheart, blurring of an image was large due to the motions. Shortening theETS down to approx. 8 msec enabled imaging of the chest endurable touse. Specifically, a shortened ETS of about 5 msec showed a superiorstabilization of images. The data acquisition SE sequence may be asequence of which an interval between echoes due to RF pulses is set toa value of not more than 8 msec. A possible reason is considered suchthat, in addition to the foregoing various types of explanation, theshortened ETS makes it possible to put the entire data acquisitionperiod within a cardiac stationary range of a heartbeat.

On one hand, making the ETS long will lead to a long data acquisitionwindow, which is possible to acquire data in a narrow band. That is, theETS can be adjusted depending on the type of object to be imaged. Forexample, if a region to be imaged is distant from the chest, MR imagesof higher diagnostic capabilities can be obtained by setting an ETS ofabout 10-15 msec or more.

(12) In addition, in the case that the size of a matrix is increased inthe phase-encoding direction, data acquisition based on the half-Fouriertechnique makes it possible to start imaging from the center or about inthe k-space concerning a desired delay time T_(D). This reduces blurringof an image to a minimum.

(13) In multislice imaging, it is avoidable to have a saturationphenomenon of signals, which is caused by excitation pulses, from flowof blood in an adjoining slice.

(14) Furthermore, the delay time T_(D) for the ECG-gating technique canbe altered or controlled arbitrarily, producing MR images preciselyreflecting the alteration or control. An appropriate delay time T_(D)may be set depending on differences in individual patients in terms ofmagnetic characteristics.

(15) Because MR contrast mediums are not used, the feature ofnon-invasiveness is also obtained. Thus, compared with thecontrast-based MRA imaging, the patient's mental and physical burdensare extremely reduced.

In this embodiment, the application step of the CHESS pulse may beomitted to compose the pre-sequence with a plurality of divided MTpulses alone.

Moreover, the MT pulse train can be set without the slice gradient pulseapplied concurrently therewith. That is, the MT pulses are applied in aslice-non-selective manner. Also, the CHESS pulse for fat suppressioncan be applied slice-selectively with a slice gradient pulse appliedconcurrently with the CHESS pulse.

The MT pulses of the MT pulse train or the RF pulse of the CHESS pulsemay be applied with a slice gradient taking account of a range to beimaged.

The command to control the breath hold period can be realized byon/off-controlled light signals instead of using sound or voice.

The delay time for optimizing the temporal location of the dataacquisition sequence SQ_(acq) may be determined by specifying a timefrom an R-wave to the front of the pre-sequence SQ_(pre).

Fourth Embodiment

Referring to FIG. 21, a fourth embodiment of the present invention willbe described. An MRI system of this embodiment features a noveltechnique for informing patients of timing of her or his breath hold.

In ECG-gating imaging, one particular problem is that patient'srespiratory body motions, which are in asynchronism with the ECG signal,deteriorates image quality. In the case of the conventionally known“segmented FFE scan,” images are improved by using another means todetect such body motions. However, this embodiment employs a breath holdtechnique, not such body-motion-detecting means, in order to suppressinfluence caused by the respiratory body motions.

In the present invention, to obtain T2-weighted image contrast premisesthe elongation of the repetition time TR to 2000 msec or more.Therefore, the TR is set to a longer period of 4000-10000 msec, and theECG gating is set to operate about every four to ten heartbeats. Thebreath hold is ordered every time the ECG gating operates, which givesan intermittent breath hold.

For the intermittent breath hold, how to inform patients of the start ofeach breath hold and its period are essential. Namely, significantmatters are how precisely the start timing is given to patients and tominimize the period of each breath hold. As one example of a minimizedperiod of one breath hold, a period of data acquisition for each singleshot scan is given as approx. 480 msec (=80+5×80) where the echo trainspacing ETS=5 msec, echo time TE(effective TE)=80 msec, and the matrixsize in the phase-encoding direction=160.

The fact that a standby period (the foregoing delay time) from atriggering pulse synchronous with an R-wave of the ECG signal to thediastole appropriate for imaging heart is relatively long; 300-600 msec,is positively used in the present embodiment. During the period, anintermittent breath hold that makes use of sound signals generated inassociation with applying gradient pulses is ordered, in addition to theapplication of the foregoing MT pulses and the CHESS pulse.

Host computer 6 and sequencer 5 in this embodiment perform a pulsesequence including a sequence for intermittent breath holds shown inFIG. 21. As shown therein, in response to a not-shown triggering pulsesynchronized with an R-wave, sequencer 5 starts to apply a pulse trainBH_(start) urging a patient to hold her or his breath in a standbyperiod T_(D). As one example, the pulse train BH_(start) consists offour pulses B₁-B₄. Each of the pulses B₁-B₄ has a pulse waveform made byrepeating N-cycles a rectangular pulse of a given duty ratio T_(duty).These pulses B₁-B₄ are applied, in turn, via the x-, y- or z-coil asX-axis, Y-axis or Z-axis directional gradient pulses G_(x), G_(y) orG_(z).

The application of these gradient pulses causes stresses owing to pulsedelectromagnetic forces in a gantry structure sustaining the x-, y- andz-coils 3 x-3 z. These forces generate sound. For example, a patientlaid in the gantry can hear a series of four intermittent sounds, likebuzzer's sounds of “boo, boo, boo, boo.” The amplitude of these soundscan be controlled by adjusting the strength of the gradient pulses,while the tune thereof can be controlled by adjusting the duty ratioT_(duty).

Immediately after these breath hold requesting sounds, the pulsesequence SQ_(pre) and SQ_(ima) formed in the same way as in the thirdembodiment is performed for one heartbeat.

The patient is asked, in advance, as a promise, for performance of anintermittent breath hold in a manner such that, when you hear theintermittent sounds start, hold your breath for about one second or so(namely, a very little time); after this, you can breathe. Inparticular, the 3D imaging requires timing of the breath hold not to beshifted. Therefore, it is preferred that the patient exercise the breathhold using the intermittent breath hold sequence shown in FIG. 21, priorto actual imaging.

Accordingly, the patient can start holding her or his breath in thecourse of the intermittent breath-hold-requesting sounds and keep thebreath-held state for a moment. A patient who is good at holding breathcan start to hold her or his breath when the first sound is generated orthe former several sounds are generated. Almost all patients can startto hold her or his breath at any time in a period during which all theintermittent sounds have continued. And the breath hold is done for awhile, during which breath-held period the pre-sequence SQ_(pre) anddata acquisition sequence SQ_(ima) are automatically and quicklyperformed. Under the performance of the pre-sequence SQ_(pre) and dataacquisition sequence SQ_(ima), some kinds of sounds different from thebreath-hold-requesting sounds are generated. Since the patient startsher or his breath hold in response to the intermittent sounds and keepsits hold state for a while, both the pre-sequence SQ_(pre) and dataacquisition sequence SQ_(ima) are included in the breath hold state.When a few seconds have passed after having been released from thebreath hold, another series of intermittent sounds are again generated,requesting the patient to perform the next breath hold.

Accordingly, the intermittent breath hold almost completely matched tothe ECG gating time produced every few heartbeats can be done bypatients. Because the breath hold is intermittent, even if the dataacquisition period is longer to some extent, patients are able to holdher or his breath easily. Artifacts caused by respiratory body motionscan be suppressed greatly. This intermittent breath hold can easily beperformed with the existing MRI system new hardware; it is not necessaryto add particular hardware. Differently from sound messages (sayingwords) that inform a patient of timing of the breath hold by a controlcomputer, there is no need for executing processing for synchronism withimaging pulse sequence, facilitating the software processing.

Thus, there is provided an MRI system by which the intermittent breathhold for a self-navigator technique for the breath hold can be performedthat is novel, prominent, and suitable for 3D ECG-gating imaging.Various advantages coming from the FSE scan are also obtained, like thethird embodiment.

Fifth Embodiment

Referring to FIG. 22, a fifth embodiment of the present invention willbe described, which provides another configuration about the foregoingintermittent breath hold technique.

Host computer 6 and sequencer 5 of this MRI system are constructed toperform a pulse sequence (including an intermittent breath-holdsequence) shown in FIG. 22. As shown therein, sequencer 5 responds to atriggering pulse (not shown) synchronizing with an R-wave of the ECGsignal. When a standby time T_(x) has passed after this response,sequencer 5 informs a patient of a period during which the patient canbreathe freely (referred to as “free breath period”) by applying a pulsetrain BH_(tempo) for preparing for the breath hold. The pulse trainBH_(tempo) is made up of, for example, seven pulses C₁-C₇, which areapplied over a few heartbeats for a vacant period in one repetition timeTR, no data acquisition being carried out in the vacant period. Each ofthe pulses C₁-C₇ has a pulse waveform made by repeating N-cycles arectangular pulse of a given duty ratio T_(duty). These pulses C₁-C₇ areapplied via the x-, y- and/or z-coil as X-axis, Y-axis and/or Z-axisdirectional gradient pulses G_(x), G_(y) or G_(z).

Like the foregoing embodiment, the application of the gradient pulsespermits the gantry to produce sounds. For example, a patient laid in thegantry can hear a series of seven intermittent sounds, like buzzer'ssounds of “boo, boo, boo, . . . , boo” as breath-hold preparing sounds.The amplitude of these sounds can be controlled by adjusting thestrength of the gradient pulses, while the tune thereof can becontrolled by adjusting the duty ratio T_(duty).

After the generation of the breath-hold preparing sounds, a givenstandby period T_(start) is set. This standby time is then followed bythe same pulse sequences SQ_(pre) and SQ_(ima) as those in the thirdembodiment. The standby time T_(start) is composed of a given waitingtime T_(w) counted from an R-wave assigned to imaging and a littlemargin T_(α).

A promise to the patient is that, when you hear the intermittent soundsstop, hold your breath for about two seconds or so (namely, a very shorttime); after this, you can breathe. In this case, it is also preferredthat the patient exercise the breath hold using the intermittent breathhold sequence shown in FIG. 22, prior to actual imaging.

Thus, the patient can be ready for a breath hold by adjusting her or hisbreath using the breath-hold preparing sounds (intermittent sounds). Thebreath hold can be done for about two seconds during or after thepreparing sounds. Therefore, most of the standby time T_(start) and thepulse sequences SQ_(pre) and SQ_(ima) are completely included in aperiod of the breath hold. After this appropriate breath-hold period,the patient can breathe freely. In succession, every time thebreath-hold preparing sounds are generated, the same breath hold isrepeated.

Accordingly, the same or similar advantages as or to those in the fourthembodiment can be obtained. There is also the advantage that theintermittent breath-hold technique is widened in variations. Thistechnique can be selected depending on individuals. The breath hold ismade sure in ECG-gating 3D imaging.

In the foregoing fourth and fifth embodiments, the intermittent breathhold is characteristic of being applied to a pulse sequence associatingwith a pre-sequence employing the MT pulse train and the CHESS pulse.Alternatively, the intermittent breath-hold technique is also applicableto other pulse sequences for 3D imaging.

Sixth Embodiment

Referring to FIGS. 23, 24A and 24B, a sixth embodiment of the presentinvention will be described. This embodiment is concerned with one usageof the intermittent breath-hold technique described above.

More specifically, this intermittent breath-hold technique is adoptedinto a 3D SPEED (Swap Phase Encode Extended Data) method.

The SPEED method, as proposed by JMRI, Miyazaki, M., et al., 98March/April, is a 2D or 3D imaging technique in which data acquisitionis performed for one image with the phase-encoding direction altered. Incase of 3D SPEED imaging, with the slice-encoding direction fixed, imagedata are acquired a plurality of times for different phase-encodingdirections. At the stage of processing image data, three-dimensional rawdata are reconstructed every phase-encoding direction, then a pluralityof sets of resultant real-space three-dimensional image data aresynthesized with each other, pixel-by-pixel, into one set ofthree-dimensional image data.

In this embodiment, pluralities of times of scans are performed with thephase-encoding and readout directions mutually swapped, but with theslice direction unchanged. FIG. 23 outlines a scan instructed by thehost computer 6 and sequencer 5.

In addition to the SPEED method, this scanning adopts ECG-gating andintermittent breath-hold techniques. A delay time T_(D) that determinesan ECG-based synchronization time is set in advance so as to bring aperiod of data acquisition into an appropriate cardiac phase.

For example, as shown in FIGS. 24A and 24B, data acquisition from athree-dimensional volume region set in the abdomen is carried out with2n-times (n is positive integer more than 1) of scans in the order ofRL_(se1), HF_(se1), RL_(se2), HF_(se2), . . . , RL_(sen), HF_(sen),under the FASE method, for example. Each of the scans RL_(se) andHF_(se) represents an ECG-gating single scan for each slice-encodingamount, which provides three-dimensional raw data of the volume region.However, the phase-encoding direction differs between the scans RL_(se)and HF_(se). For the scan RL_(se), the phase-encoding direction is setin the lateral direction of the patient's body, as shown by a solid lineX1 in FIG. 24B. In contrast, for the scan HF_(se), the phase-encodingdirection is set in the longitudinal direction of the patient's body, asshown by a solid line X2 in FIG. 24B, which is different from thedirection X1 by 90 degrees. The subscripts se₁, . . . , se_(n) showamounts of slice-encoding for each scan. According to the exemplifiedsequence shown in FIG. 23, the first and second ECG-gating scans areeach performed every slice-encoding amount se₁ (to se_(n)).

In imaging the chest and abdomen portion, the breath hold is absolutelynecessary in order to prevent respiratory artifacts. However, because ittakes a long time to perform a three-dimensional scan, it is difficultto cover the whole imaging time by one time of breath hold. The breathhold is, therefore, carried out intermittently. This intermittent breathhold is characteristic of informing a patient of timing distinguishing abreath-hold period T_(bh) (i.e., data acquisition period) and a freebreath period T_(sp) with sounds caused by applying gradients.

In the case of FIG. 23, such distinguishing timing is determined suchthat one pair of the scans RL_(se1) and HF_(se1) is made and thebreath-hold period T_(bh) is assigned to a period during which one pairof scans last. The free breath period T_(sp) is assigned to thefollowing period lasting to the next data acquisition (for example, inthe case of a period between the scans HF_(se1) and RL_(se2)). This freebreath period T_(sp) is informed to a patient by breath-hold preparingsounds generated by applying gradients. Like the foregoing embodiments,the preparing sounds are generated by applying a gradient pulse G_(sd),. . . , G_(sd) in the X-, Y- and/or Z-axis directions. The breath-holdpreparing sounds can be heard intermittently generated at intervals,like buzzer's sounds “boo, boo, . . . , boo.”

The amplitudes and tones of the breath-hold sounds can be controlledappropriately by adjusting parameters concerning the gradient pulsesG_(sd). It is preferred that intervals of a patient's respiration (thepace of respiration) be examined in advance in her or his natural state,and each interval in the breath-hold preparing sounds (that is, thenumber of sounds produced during the free breath period T_(sp)) be madeto agree with each respiration interval.

A single set of three-dimensional raw data whose phase-encodingdirection is set laterally are reconstructed into a set of image data,while another single set of three-dimensional raw data whosephase-encoding direction is set longitudinally are also so. Both thethree-dimensional image data thus-reconstructed are synthesizedpixel-by-pixel into a new one set of three-dimensional image data. Foreach slice encode, two frames of data whose phase-encoding directionsdiffer from each other undergo MIP processing, providing a finalthree-dimensional MRA data. The image synthesizing processing may besimple adding or averaging.

According to this embodiment, the ECG-gating timing (i.e., delay timeT_(D)) is optimized beforehand, scanning can be performed so as toacquire echo signals having the highest intensities from flow of blood.It is firmly avoidable that echo signals are lowered or become almostzero on account of relatively slow velocities of flow of blood oroccurrence of a flow-void phenomenon. The optimized synchronizationtiming produces MRA images of a stable and higher depiction performance.Use of FSE-system pulse sequences is advantageous in susceptibility anddistortions of contours.

Moreover, final image data are produced from data acquired a pluralityof times with altered phase-encoding directions. Because of altering thephase-encoding directions, the image data thus acquired are superior inthe depiction performance, especially when flow of blood whose T2relaxation time is shorter is imaged. This is because effects that pixelvalues are enhanced (or blurred) in the phase-encoding direction can beutilized. Pulmonary blood vessels that run in all directions aredepicted with higher ratios of S/N and higher contrast degrees ofparenchyma, providing finer bits of information about running directionsof blood vessels.

Concerning the intermittent breath-hold technique informing patients ofthe breath-hold and free breath periods, following advantages can beobtained.

First, only the gradient pulses G_(ds), . . . , G_(ds) are added to theimaging pulse sequence, and there is no need for installing specialhardware units. The processing for controlling sound generation isrelatively easier. It is easy to synthesize both the imaging pulsesequence and the ECG signal.

Secondly, the tones, magnitudes and intervals of the breath-holdpreparing sounds can be easily adjusted by parameters about the pulsesG_(ds), . . . , G_(ds). Particularly, the intervals of the intermittentsounds can previously be set to the pace of a patient's naturalrespiration. In that case, it is easier to capture the start timing ofthe breath-hold period next to a free breath period. Namely, a patientcan breathe in agreement with the breath-hold preparing sounds(intermittent sounds) in a free breath period, and starts a breath holdwhen the last one of the preparing sounds is heard. The number ofpreparing sounds will be taught to a patient in advance or the lastsound will be differentiated in tone from the other ones. Thus,instructions for the breath hold are easy to be understood, resulting inthat a patient can enter a steady breath-hold period.

The steady and stable breath hold allows artifacts caused by therespiratory motions to be reduced.

Even if automatic sound massages saying words are generated togetherwith this intermittent sound technique, the intermittent breath hold canbe easier, because the number of breath-hold preparing sounds definefree breath periods.

The third advantage of this embodiment is that operator's work requiredfor the breath hold is noticeably reduced thanks to this self-navigatortechnique for the breath hold. The efficiency of operation and a patientthroughput are improved.

Seventh Embodiment

Referring to FIG. 25, a seventh embodiment of the present invention willbe explained. This invention also concerns another example of theintermittent breath hold.

This embodiment provides an imaging technique in which the intermittentbreath hold is incorporated into such three-dimensional imaging as3D-MRCP (MR cholangiopancreatography). In the 3D imaging, theintermittent breath hold is absolutely necessary, because of its longscan time. Particularly, to depict entities having longer T2 relaxationtimes, needs the repetition time to be longer. Thus, it is difficult tocomplete the entire imaging within one time of breath hold.

Therefore, as shown in FIG. 25, a breath-hold period T_(bh) and a freebreath period T_(sp) are repeated every time when the slice-encodingamount SE is altered. The free breath period T_(sp) is assigned to theintermittently remaining intervals other than periods (breath-holdperiods T_(bh)) when a pulse sequence for data acquisition is applied. Apatient is informed of these intermittent periods T_(sp) usingbreath-hold preparing sounds generated by applying gradient pulsesG_(Sd), G_(Sd). In this embodiment, the first breath-hold period T_(bh)is set to a data acquisition period for the slice-encoding amount SE₁,and a remaining interval of the repetition time TR is the first freebreath period T_(sp). Like this, the periods are repeated.

Control of the breath-hold preparing sounds is done in the same way asin the sixth embodiment. The control is united with imaging control ofthe pulse sequence, and host computer 6 and sequencer 5 operatecooperatively to perform the imaging control.

Each set of 3D raw data acquired every slice-encoding amount with theforegoing intermittent breath hold are subject to a 3D Fourier-transformin the arithmetic operation unit 10, providing 3D real-space image data.

Therefore, the equivalent advantages to the sixth embodiment areobtained.

In the sixth and seventh embodiments, MR angiography was an object to beimaged, but it is not limited to only blood vessels. For example, afilamentous tissue may be another objective. In particular, if anobjective has a longer T2 relaxation time, such objective can be imagedwell.

Imaging methods to which the intermittent breath hold of the presentinvention is applied are not restricted to the ones mentioned above. Forexample, dynamic imaging can be done using such breath hold technique.The dynamic imaging often requires that several times of imaging beperformed before and after injecting a contrast medium, respectively. Inthis case, it is preferred that a patient be informed of a period forthe free breath (in other words, timing of starting breath holds) by theforegoing preparing sounds generated by applying the gradient pulses, asdescribed above. Still, the intermittent breath hold of the inventioncan also be adapted to discontinuous imaging methods, such as segmentedFFE (Fast FE) preferable to imaging of the heart.

Moreover, as means for detecting the cardiac temporal phases, theforegoing ECG-related elements may be replaced by any other means, suchas gating means to uses pulses detected from a finger of a patient,gating means to use echo signals themselves, or others.

Although the description above contains many specificities, these shouldnot be construed as limiting the scope of the invention, but rather asmerely providing illustrations of some of the presently preferredembodiments of this invention. Thus, the scope of this invention shouldbe determined by the appended claims and their equivalents.

What is claimed is:
 1. A magnetic resonance imaging (MRI) systemproviding an MR image of an imaging region of an object, the systemcomprising: an MT (magnetization transfer) pulse applying unitconfigured to apply a plurality of divided MT pulses, each of thedivided MT pulses having a same flip angle; an acquiring unit configuredto acquire an MR signal from the imaging region after applying theplurality of divided MT pulses; and an image producing unit configuredto produce the MR image using the acquired MR signal.
 2. The MRI systemof claim 1, wherein the plurality of divided MT pulses is divided suchthat a flip angel of each of the plurality of divided MT pulses becomessmaller than a flip angel of a non-divided single MT pulse.
 3. The MRIsystem of claim 1, further comprising an arithmetic operation unitconfigured to process an arithmetic operation pixel-by-pixel on the MRimage produced by the image producing unit, and to generate an imagedifferent from the produced MR image.
 4. The MRI system of claim 3,wherein the arithmetic operation unit performs a difference operationpixel-by-pixel between a first MR image and a second MR image, the firstMR image being obtained while the plurality of the MT pulses is turnedoff, and the second MR image being obtained while the plurality of theMT pulses are turned on.
 5. The MRI system of claim 1, wherein nogradient pulse is applied during application of the plurality of dividedMT pulses.
 6. The MRI system of claim 1, wherein a single continuousgradient pulse is applied during application of the plurality of dividedMT pulses.
 7. The MRI system of claim 1, wherein a plurality of separategradient pulses is respectively applied to the plurality of divided MTpulses.